Process for electrolysis of alkali metal chloride



Dec. 29, 1970 N, JR ETAL 3,551,309

PROCESS FOR ELECTROLYSIS 0F ALKALI METAL CHLORIDE Filed Dec. 4, 1967Alla/l V kombk uom FI/ter Deco/"poser 4 ll k&-2 \0\-\uhQn F C L! 3. c aU. r al 0/ f/ 6 6 r 5 in. 00 m. E 1

INVEN TOR.

United States Patent Office 3,551,309 Patented Dec. 29, 1970 ABSTRACT OFTHE DISCLOSURE A process for the electrolysis of an alkali metalchloride brine in a mercury cathode electrolytic cell using a portion ofthe dechlorinated brine for the acidification of the resaturated,purified brine being returned to the cell.

This invention pertains to a process for the electrolysis of an alkalimetal chloride in a mercury cathode cell. More particularly it pertainsto an electrolysis process employing particular conditions and steps tominimize the acid requirement for the process.

In the operation of mercury cathode cells, substantially saturated brineis circulated through the cell where it is passed between a fixed anodeand a flowing mercury cathode to subject it to electrolysis. Only asmall fraction of the alkali metal chloride in the brine is electrolyzedin passing through the cell. The depleted brine discharged from thecell, while still highly concentrated, is resaturated by being contactedwith solid chloride salt and returned to the cell. In addition to theelectrolysis of the alkali metal chloride, other reactions take place inthe electrolytic cell. Hydrochloric acid is formed in the cell by someof these reactions but is generally lost through further side reactions.

Since it is essential in the electrolysis to use a brine substantiallyfree of impurities, such as iron, magnesium and others, the brine, afterresaturation, is treated to remove the impurities which it may havepicked up in being contacted with the solid salt. A purification methodcommonly used to remove the impurities is to precipitate them underalkaline conditions. Addition of an alkali such as sodium or potassiumhydroxide or carbonate has to be made to the brine, since the depletedbrine is usually at a pH of 2-3 upon being discharged from the cell.After the removal of the precipitated impurities, the purified brine isacidified prior to the introduction into the cell. Thus, considerablequantities of alkali and acid are used in the process.

It is, therefore, an object of this invention to provide an improvedprocess for the electrolysis of an alkali metal chloride. Another objectis to provide a process wherein the amount of chemicals used in thetreatment of the brine is substantially reduced. A further object is tomore elfectively utilize the acid generated in the cell duringelectrolysis.

The above and other objects are attained, according to the invention, bysaturating and purifying only a portion of the dechlorinated brine andrecycling a portion of the dechloroinated brine to the cell feed toacidity the resaturated brine. The brine flow rate through the cell isincreased proportionately to the amount of recycle to maintainsubstantially the same chlorine production capacity. The brine isacidified in passing through the cell by the dissolution of chlorine. Aportion of the hydrochloric acid thus formed is utilized by the recycleto acidify the resaturated portion of the brine. The recycle may beincreased to the extent that the combined portions are at the propercell feed acidity.

Further, additional advantages are attained, according to the invention,by operating the cell under controlled conditions of impurity levels andflow rates of the brine in the cell. In operating the cell under thecontrolled conditions, the undesirable side reactions are minimized. Asthe result, the brine feed does not have to be acidified to the extentotherwise necessary, and the acid formation in the cell is utilized to agreater extent to lower the pH of the brine to its final level.

The invenion will be explained in more detail in conjunction with theattached drawing which is a schematic flow diagram illustrating anembodiment of the invention. In the process shown in the drawing, asubstantially saturated brine, such as a sodium or potassium chloridebrine, is fed to the electrolytic cell at an inlet temperature of 60 toC. and passed through the cell electrolyzing some of the alkali metalchloride in the brine. The electrolysis is effected at a rate such thatthe temperature rise obtained of the brine in the cell is in the rangeof from 0.12 to 040 C., per foot of cell length. The depleted brinedischarged from the electrolytic cell is passed to Dechlorinator 4.While the dechlorinator is shown as one block, it may consist of anumber of units. For example, the depleted brine after coming from thecell may be subjected to a preliminary dechlorination by being subjectedto a partial vacuum and then further dechlorinated, as by air stripping,to reduce the chlorine content to less than ten parts of chlorine permillion parts of brine. In the dechlorination operation, hydrochloricacid may be added, if the concentration of hypochlorous acid in thedepleted bine is excessive or the brine contains chlorate. However,generally, in the operation of the cell under the controlled conditionsof impurity levels and brine flow rates, this addition is not necessary.The depleted brine is substantially free of chlorate and the pH of thebrine is usually in the range of 2.0 to 2.5 which is sufficient for theremoval of the limited about of hypochlorous acid found in the brine.

From the dechlorinator, the dechlorinated brine is divided into twoportions. One portion is recycled to storage tank 5, while the otherremaining portion is passed to saturator 6 where it is contacted withsolid alkali metal chloride and is resaturated. The portion recycled isgenerally about 20 to percent of the amount of brine being resaturated.

Prior to resaturation, the portion of the brine to be resaturated isusually neutralized by the addition of an alkali, such as the hydroxideor carbonate of the same alkali metal as in the brine. The pH of thebrine does not change appreciably on dechlorination, especially if thebrine does not contain large amounts of hypochlorous acid. Thus, thebrine leaves the dechlorinator at a pH in the range of 2 to 2.5 or closeto that of the depleted brine. A sufficient amount of the alkali may beadded ahead of the saturator just to neutralize the brine to a pH of 7prior to contact with the salt. On the other hand, sufficient alkali maybe added to raise the pH to the level desired in the later purificationstep. When the brine to the saturator is only neutralized to a pH of 7or so, further addition of alkali is made to the saturated brine in thepurification step. The impurities precipitated out under alkalineconditions are removed by settler 7 and filter 8. The purified,saturated brine is then intermixed in storage tank 5 with the recycleddechlorinated brine which was not subjected to the saturation andpurification steps. Before returning the combined portions of the brineto the cell as feed, the brine from storage tank 5 is passed throughheat exchanger 9 to bring the brine to the cell inlet temperature.Relatively small amounts of hydrochloric acid may be added to the brinefeed to make the final adjustment to bring the pH to the acidity usuallyemployed. In the operation of the cells under the controlled conditions,normally a higher pH, in the range of 4.5 to 6.5, is used. The chlorineformed in the electrolysis is withdrawn from the cell and processedfurther while is not shown on the drawing. The amalgam formed iscontacted with water in Decomposer where the alkali metal amalgam reactswith the water to form a hydroxide solution and hydrogen. The mercuryfrom the decomposer is returned to the cell.

Since, generally, the electrolytic cells are operated at at relativeconstant current density, the concentration of the alkali metal in themercury is primarily dependent on the rate of flow of mercury throughthe cell. The mercury being returned from the decomposer usuallycontains less than 0.010 weight percent of the alkali metal. Thus, therate of flow of mercury through the cell is preferably maintained atrates normally used such that the concentration of the alkali metal inthe mercury at the exit is in the range of from 0.20 to 0.25 weightpercent. However, at times the maximum concentration of the alkali metalin the mercury may be increased to about 0.30 or even 0.35 weightpercent for a short period of time.

The flow rate of the brine through the cell is generally increased by anamount proportional to the recycle used to obtain about the samechlorine production as without the recycle. Under the controlledconditions, a flow rate of about from 8 to 14 gallons per minute perfoot of width of the cell, preferably in the range from 9 to 11 gallonsper minute per foot, is employed. Electrolytic cells are relativelyflexible in this respect and can be operated under various brine flowrates. Since the brine in the cell flows through passages in the cellother than just the passage or gap between the anode and the cathode,the velocity of the brine may vary in difierent parts of the cell. Thus,the flow rates are expressed in volume per unit time per linear foot ofcell width. The flow rates, as expressed, are relatively indicative ofthe velocities obtained in the cell. However, an increase in the flowrate increases the brine level in the cell so that the passage of theadditional brine through the cell may not be due entirely to increasedvelocity.

By increasing the rate of flow of the brine, other beneficial resultsare obtained in addition to the advantages of the physical effects ofthe higher velocity. The higher brine level in the cell is advantageous.Also, a lower temperature gradient of the brine in the cell is obtainedpermitting a more uniform operation of the cell which improvesefiiciency. While the discharged temperature of the brine from the cellmay be widely varied, there is a practical limit above which it isundesirable to operate the cell due to the high vapor pressure of water.This results in a large amount of water being drawn off with thechlorine which has to be processed later to be removed. Thus, in theoperation of the cell, a higher inlet temperature may be used withincreased flow rates of the cell without exceeding the practicaldischarge temperature.

A sufiicient amount of the decholrinated brine is subjected toresaturation and purification steps to dissolve in the saturator theamount of solid chloride salt which was depleted from the total brine inpassage through the cell. The actual amount of the dechlorinated brinerequired may vary somewhat from the amount generally used without therecycle, since the concentration desired of the alkali metal chloride inthe brine is at a higher level to compensate for the dilution efiFectobtained by the combination of this stream with the less concentratedrecycle stream. Generally, the temperature increase obtained during theelectrolysis of the brine sufiiciently increases the solubility of thealkali metal chloride so that the required amount of salt can bedissolved at the higher concentration level with about the same orsomewhat less amount of brine used Without recycle. The solubility ofthe alkali metal chloride may be further increased by heating theportion of brine to be resaturated by means of an external heater.However, this additional heating is usually unnecessary. Generally, thebrine may be resaturated to a sufiiciently high concentration withoutexternal heating to maintain a substantially saturated brine, having aconcentration in the range of 295 to 310 grams of the alkali metalchloride per liter of brine, with a recycle ratio of 40 to percentpreferably used. This amount or ratio of recycle is sufiicient to bringthe pH of the saturated, purified portion down to a pH close to the pHused for the brine feed. Since the pH of the dechlorinated brine is inthe range from 2 to 2.5, the portion of the dechlorinated brine beingrecycled is sufiicient to acidify or lower the pH of the treated brineto about the degree necessary. However, if the amount of dechlorinatedbrine recycled is not sutficient to lower the pH to the level desired,an acid may be added to supplement the dechlorinated brine in theacidification. Even though a sufiicient amount of dechlorinated brine isavailable for recycle to effect the desired acidification, it is oftendesirable, for operational purposes, to use a small amount ofhydrochloric acid for the final control of the pH.

It is apparent that the amount of dechlorinated brine recycled or theacid required to acidify the saturated, purified brine will depend uponthe conditions used in the purification of the brine. Using the usualmethods for the purification of the brine, the brine may be subjected todifierent degrees of alkalinity depending upon the impurity orimpurities to be removed. Generally, a pH in the range of 9 to 12 may beused; however, in the presence of certain impurities, for examplealuminum, part of the treatment may be effected at a pH of 8.5 or lower.In the treatment employed, the impurity level in the resaturated,purified portion of the brine is reduced to a point such as to maintainthe desired low impurity level in the feed brine after the addition ofthe recycle portion of the dechlorinated brine. Under the controlledconditions, the impurity levels in the cell feed are maintained suchthat the listed metals are present in less than the following parts permillion parts of brine:

Chromium .01 Molybdenum .01 Vanadium .01 Titanium .10 Iron 10 Aluminum1.0 Magnesium 3 .0 Calcium 15.0

However, it may be desirable to further lower the concentration of someof these metals in particular combinations. Some metals, such as cobalt,nickel, iron, copper, aluminum and magnesium may function as promotersand have synergistic effect with other metals. The presence of thesepromoter metals may require the lowering of the concentration of theaffected metal by at least the concentration of the promoter.

In the operation of a plant, DeNora type electrolytic cells wereoperated according to the invention for a period of 24 hours. A recycleof dechlorinated brine equal to about 78 percent of the amount of brinebeing resaturated and treated was maintained. A brine flow rate throughthe cells of about 10.3 gallons per minute per foot of cell width wasemployed. The brine feed containing about 297 grams of sodium chlorideper liter was charged to the cells at a temperature of about 66 C. andat a pH of about 6.1. The depleted brine containing about 270 grams ofsodium chloride per liter was discharged from the cell at a temperatureof about 79 C. and at a pH of 2.2. Upon leaving the cell, the depletedbrine was dechlorinated by evacuation and also by air stripping todecrease the chlorine content of the brine from about 80 parts of freechlorine per million parts of brine to about 1 part per million. The pHof the dechlorinated brine was about 2.4.

The portion of the dechlorinated brine from the air strippers which wasto be resaturated was neutralized with caustic to a pH of about 10.5 andresaturated without further heating to about 318 grams of sodiumchlochlorine produced by the plant was about 4 pounds per ton.

In operation of the plant without the recycle, the brine was passedthrough the cell at a flow of 6.8 gallons per minute per foot of cellwidth. The depleted brine was dechlorinated as above and passed throughthe resaturator and purified to the level above prior to being returnedthrough the cell. The hydrochloric acid requirements were about 105pounds per ton of chlorine produced.

The hydrogen content of the chlorine gas stream was about 0.1 volumepercent during both of the operations.

What is claimed is:

1. In the electrolysis of a substantially saturated alkali metalchloride brine in a mercury cathode electrolytic cell, wherein the brineis passed through the cell to electrolyze the alkali metal chloride inthe brine at a rate to obtain a particular chloride production at apredetermined current density; the depleted brine, discharged from thecell after passage through the cell, is contacted with solid salt toresaturate the brine; the resaturated brine purified under alkalineconditions; and the resaturated, purified brine acidified and returnedto the cell; the improvement which comprises dechlorinating the depletedbrine; recycling a portion of the dechlorinated brine to the cell feed;resaturating the remaining portion of the dechlorinated brine at atemperature such as to dissolve the amount of alkali metal chloridedepleted from the total brine in passing through the cell; purifying theresaturated brine under alkaline conditions; combining the resaturated,purified portion of the dechlorinated brine with the recycled portion ofdechlorinated brine to acidify the resaturated, purified portion of thebrine; and passing the combined portions through the cell at a flow rateincreased proportionately to the amount of recycle to obtainsubstantially the same chlorine production from the cell at thepredetermined current density.

2. In the electrolysis of a substantially saturated a1- kali metalchloride brine in a mercury cathode electrolytic cell, wherein a feedbrine is passed 'to a cell at a particular inlet temperature; thedepleted brine, discharged from the cell at an increased temperatureafter passage through the cell, is contacted with solid salt toresaturate the brine; the resaturated brine purified under alkalineconditions; and the resaturated, purified brine acidified and recycledthrough the cell; the improvement which comprises dechlorinating thedepleted brine, recycling a portion of the dechlorinated brine to thecell feed, neutralizing the remaining portion of the dechlorinatedbrine;

resaturating the neutralized portion of the dechlorinated ibrine at atemperature such as to dissolve the amount of alkali metal chloridedepleted from the total brine in passing through the cell; purifying theresaturated portion of the brine under alkaline conditions; combiningthe resaturated, purified portion of the dechlorinated brine to acidifythe resaturated, purified portion of the brine; and passing the combinedportions of the brine as feed brine to the cell at the inlettemperature, said portion of dechlorinated brine recycled being in therange of to 100 percent of the remaining portion of the dechlorinatedbrine neutralized, resaturated, and purified.

3. A process according to claim 2 wherein the alkali metal chloridebrine is a sodium chloride brine.

4. A process according to claim 3 wherein the recycled portion of thedechlorinated brine is from 40 to 80 6 percent of the portion of thedechlorinated brine neutralized, resaturated, and purified.

5. In the electrolysis of a substantially saturated alkali metalchloride brine in a mercury cathode electrolytic cell, wherein a feedbrine is electrolyzed in the cell by passage through the cell at a rateto obtain a particular chlorine production; the depleted brine,discharged from the cell after passage through the cell, is contactedwith solid salt to resaturate the brine; the brine purified underalkaline conditions; the resaturated, purified brine acidified; andreturned to the cell; the improvement which comprises passing the feedbrine through the cell at a rate of 8 to 14 gallons per minute per footof cell width to electrolyze the alkali metal chloride at a rate suchthat a temperature rise obtained in the cell of the brine' is in therange of 012 C. to 0.40 C. per foot of cell length; dechlorinating thedepleted brine until the chlorine content is reduced to at least 10parts of chlorine per million parts of brine; recycling a portion of thedechlorinated brine to the cell feed; neutralizing the remaining portionof the dechlorinated brine; contacting the neutralized dechlorinatedportion of the brine with a solid alkali metal chloride salt at atemperature to dissolve the amount of salt depleted from the total brinein passage through the cell; purifying the resaturated brine underalkaline conditions; combining the resaturated, purified portion of thebrine with the recycled portion of the dechlorinated brine to acidifythe purified portion of the brine; and returning the combined portionsas feed brine to the electrolytic cell at a pH in a range of 4.5 to 6.5;said resaturated portion of the brine being purified to the extent thatthe feed brine returned to the cell contains less than the followingparts of the listed metals per parts per million of brine;

Chromium 0.01 Molybdenum 0:01 Vanadium 0.01 Titanium 0.1 Iron 0.10Aluminum 1.0 Magnesium 3.0 Calcium 15.0

6. A process for the electrolysis of an alkali metal chloride brine in amercury cathode electrolytic cell, which comprises charging asubstantially saturated feed brine to the electrolytic cell at an inlettemperature in the range of 60 to C.; passing the brine through the cellat a flow rate of 8 to 14 gallons per foot of cell width to therebyelectrolyze the alkali metal chloride in the brine to obtain a depletedbrine; said electrolysis being at a rate such that a temperature riseobtained in the cell of the brine is in the range of O.l2 to 0.40 C.,per foot of cell length; dechlorinating the depleted brine until thechlorine content is reduced to at least 10 parts of free chlorine permillion parts of brine, recycling 2. portion of the dechlorinated brineto the cell feed; neutralizing the remaining portion of thedechlorinated brine; said portion of dechlorinated brine recycled beingin an amount in the range of 20 to percent of the remaining portion ofthe dechlorinated brine neutralized; contacting the neutralizeddechlorinated portion of the brine with a solid alkali metal chloridesalt at a temperature to dissolve the amount of salt depleted from thetotal brine in passage through the cell; purifying the resaturated brineby pre cipitation of the impurities under alkaline conditions; combiningthe resaturated purified portion of the brine with the recycled portionof the dechlorinated brine to acidify the purified portion of the brine;and passing the combined portions as feed brine to the electrolytic cellat a pH of from 4.5 to 6.5; said resaturated portion of the brine beingpurified to the extent that the feed brine returned to the cell containsless than the following parts of the listed metals per million of brine;

Chromium 0.01 Molybdenum 0.01 Vanadium 0.01 Titanium 0.1

Iron 0.1

Aluminum 1.0 Magnesium 3.0 Calcium 15.0

7. A process according to claim 6 wherein the brine is a sodium chloridebrine.

8. A process according to claim 7 wherein the brine is passed throughthe cell at a .rate in the range of 9.5 to 12 feet per minute per footof cell width.

9. A process according to claim 8 wherein the portion of thedechlorinated brine recycled is in the range of 40 to 80 percent of theamount of the remaining portion of the dechlorinated brine neutralizedand resaturated.

References Cited UNITED STATES PATENTS Goerg 20499 OTHER REFERENCES 10Angel et al., J. of the Electrochemical S0c., November 1952, pp.435-441.

T, TUNG, Primary Examiner US. Cl. X.R. 204125 UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 3, 551, 309 Dated December29, 1970 'Invent fl Irrther L. Dunn. Jr. and Norval l larznusson It iscertified that error appears in the above-identified patent and thatsaid Letters Patent; are hereby corrected as shown below:

Column 5, line 6 4, after "brine' inser*t--with the recycled portion ofthe dechlor'inated brine--.

Sigued and sealed this 30th day of March 1971.

(SEAL) Attest:

EDWARD M.FIE TCHERJR. Attesting Officer WILLIAM E. SCHUYLER, JR.Commissioner of Patents

